It has been highlighted that metal beryllium pebbles (pebble-like metal beryllium) are used for a neutron multiplying material in a nuclear fusion reactor blanket.
This is due to the fact that, in the nuclear fusion reactor blanket, although formation of one tritium requires one neutron, the collision of one neutron to metal beryllium causes two neutrons to be formed; therefore, use of metal beryllium pebbles for a blanket material enables tritium to be effectively multiplied, which results in expectation of advantageous improvement in nuclear fusion reaction fuel cycle.
Also, such metal beryllium pebbles are useful for moderators and reflectors of neutron in a nuclear fusion reactor blanket.
Further, such metal beryllium pebbles are expected to be employed for aerospace structural materials and the like, by utilizing the light weight and the high melting point properties.
As a method of producing such metal beryllium pebbles, there has been known a method of reducing beryllium fluoride by magnesium (hereinafter referred to as "magnesium reducing method").
The magnesium reducing method, which was developed in the United States of America and in other countries as a method of industrially extracting metal beryllium, is to produce pebble-like metal beryllium by the use of the following reaction formula: EQU BeF.sub.2 +Mg.fwdarw.Be+MgF.sub.2
In the above-mentioned magnesium reducing method, the pebble-like metal beryllium is formed in the beryllium fluoride solution, and then floats on the liquid surface of molten beryllium fluoride by the gravity concentration. Thus obtained metal beryllium pebbles are generally not less than 5 mm in particle diameter.
Besides, the metal beryllium pebbles produced by the magnesium reducing method are intermediate products obtained when the metal beryllium is industrially extracted, each of which includes various kinds of impurity elements. In particular, it includes fluorine, magnesium and the like as volatile impurities in large amounts, which possibly causes corrosive gasses to generate. Moreover, the shape of the pebble is not spheric at all, thereby lowering the packing density in the actual device, which reduces the neutron multiplying power to be satisfactorily expected.
Accordingly, in order to solve the above-mentioned problems in the magnesium reducing method, there has been developed a method called rotational electrode process (Japanese Patent Laid-open No. 3-226508, Japanese Patent Laid-open No. 6-228674, and the like).
The rotational electrode process comprises the steps of making an arc or a plasma between a plasma dissoluble electrode and a cylindrical column-like metal beryllium consumable electrode, both of which are disposed in a closed container filled with an inert gas, to thereby melt the leading end of the consumable electrode due to the heat generated by the above arc or plasma, while splashing beryllium droplets due to the centrifugal force caused by the rotation of the consumable electrode to thereby solidify the beryllium droplets in the inert gas atmosphere, providing spherical beryllium pebbles.
The beryllium pebbles obtained by the above process have various advantages. They are not only smaller and more uniform in particle diameter but they are also higher in purity and in sphericity, while being smoother in surface roughness, compared with those obtained by the magnesium reducing method.
Such metal beryllium pebbles effectively function as the neutron multiplying member, as described above; however, radiation of neutrons to the metal beryllium causes helium to generate and then condense in the crystal, which results in a volume expansion called "swelling".
Such a volume expansion causes the metal beryllium pebbles to be cracked or broken, which often lowers the resistance against external stress, thermal conductivity and the like.
The beryllium pebbles obtained by the above-mentioned rotational electrode process are excellent in anti-swelling property when compared with those obtained by the magnesium reducing method.
To solve the above-mentioned problems, the inventors have developed a technology for preventing a volume expansion of the pebbles by storing helium in a vacancy disposed in the metal beryllium pebbles, as disclosed in Japanese Patent Laid-open No. 6-228673.
The above-mentioned technology effectively prevents occurrence of the swelling; however, the tritium generated in the beryllium also is stored in the vacancy of the pebble, which thus necessarily lowers the tritium emission amount.
On the other hand, there has recently become a target to reduce the storage amount of the tritium occurring in the beryllium, to improve the tritium emission power.
Besides, the metal beryllium pebbles are expected to be employed for aerospace structural materials and the like, by utilizing their light weight and their high melting point properties. Such use requires high crash strength and excellent heat transmitting properties. However, conventional metal beryllium pebbles do not have satisfactory characteristic values for use in aerospace structural materials.